Image forming apparatus

ABSTRACT

An image forming apparatus according to the present invention irradiates a polygon mirror with laser beams from different directions, allows the polygon mirror to reflect the laser beams toward different optical axis directions to perform main scanning operation for the laser beams at a predetermined deflection angle. First and second beam detectors are disposed at the beam incident ends in the scanning direction of first and second light scanning sections to detect the start of light beam scanning operation such that the second beam detector detects it at an earlier timing than the first beam detector does. The detection result of the first beam detector is used to set sampling start timing in common between first and second image data. Based on the detection results of the first and second beam detectors, first and second image data are written into a memory, and the first and second laser source sections are controlled based on the image data read out from the memory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus such as acopy machine, printer, or fax machine and, more particularly, to amultibeam image forming apparatus that exposes a photoconductor with aplurality of laser beams to form an image.

2. Description of the Related Art

As an example of an apparatus that exposes a photoconductor with aplurality of laser beams to form an image, a multibeam image formingapparatus using a polygon mirror is known. The image forming apparatusgenerates a plurality of laser beams at a time using the polygon mirrorto exposure-scan a photoconductor drum. In forming a color image, theimage forming apparatus exposes a plurality of photoconductors spacedapart from one another with a given interval with laser beams and, afterdevelopment, transfers the color image onto a paper fed by a feedingbelt.

In the case where a plurality of laser beams are used to form a colorimage as described above, exposure scanning operations for respectivecolors must be synchronized. If the operations are out ofsynchronization, image distortion, color misalignment, or linemisalignment occurs. To cope with this problem, a light sensor (alsoreferred to as BD sensor) that detects a laser beam is disposed at astart position of scanning operation of the polygon mirror for the laserbeam to obtain a synchronization signal (Beam Detect signal, alsoreferred to as BD signal) serving as a criterion for starting thescanning operation and writing start timing of laser beam is determinedfor each color based on the synchronization signal.

Actually, however, displacement of optical elements constituting anoptical system, a change in rotation balance resulting from microvibration of rotator components due to increase in temperature of thepolygon mirror, and the like cause the timing detected by the BD sensorto fluctuate, making it difficult to perform an accurate detection.Considering BD signal obtained from a first laser beam as a reference,if the phase of BD signal obtained from a second laser beam is shiftedback and forth, the writing start timing of laser beams corresponding torespective colors becomes out of synchronization, causing colormisalignment. In some cases, starting positions of scanning operationfor photoconductors corresponding to respective colors are shifted byone line each.

An image forming apparatus using a plurality of laser beams thatprevents misalignment in the start timing of image exposure is disclosedin Jpn. Pat. Appln. Laid-Open Publication No. 2004-98449.

The image forming apparatus disclosed in the publication detectsreference points of line scans of a plurality of laser beams with alight sensor, generates synchronization signals corresponding torespective laser beams, and determines the start timing of imageexposure based on the synchronization signals. After that, the imageforming apparatus determines one reference synchronization signal fromamong the synchronization signals and uses a delay means to delayanother synchronization signal whose generation timing is close to thereference synchronization signal to make a timing difference betweensynchronization signals large, thereby determining the start timing ofimage exposure.

In this example, it is necessary to determine a synchronization signalthat is different from the reference synchronization signal and whosegeneration timing is close to the reference synchronization signal.Further, a delay means is used to make a timing difference large,causing color misalignment in some cases.

Further, Jpn. Pat. Appln. Laid-Open Publication No. 2004-98299 disclosesan image forming apparatus using a plurality of laser beams thatdetermines BD signal obtained from a first laser beam as a reference atthe time of generation of a horizontal synchronization signal (BDsignal), previously measures a phase difference between the reference BDsignal and BD signal obtained from another laser beam, and determinesthe write timing of an image according to the measured phase difference.

In this example, it is necessary to previously measure the phasedifference. Further, the phase difference is varied in some cases due todisplacement of optical elements constituting an optical system,temperature change, or mechanical micro vibration of a polygon mirror.Thus, the measured phase difference does not always correspond to theestimated value, causing color misalignment in some cases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a configuration of a light scanning unitof an image forming apparatus according to an embodiment of the presentinvention;

FIG. 2 is a perspective view showing the configuration of the lightscanning unit of an image forming apparatus according to the embodimentof the present invention;

FIG. 3 is an explanatory view showing a configuration of an imageforming apparatus using the light scanning unit;

FIG. 4 is a block diagram for explaining an image processor of the imageforming apparatus according to the embodiment of the present invention;

FIG. 5 is a block diagram for mainly explaining a laser controller shownin FIG. 4;

FIGS. 6( a) to 6(e) are timing chart for explaining operation of asynchronization system in the image forming apparatus according to thepresent invention;

FIGS. 7( a) to 7(e) are timing chart for explaining operation of asynchronization system in a general type image forming apparatus; and

FIG. 8 is a block diagram for explaining an image processor of an imageforming apparatus according to another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus ofthe present invention.

An image forming apparatus according to the present invention isapplicable to a copy machine, a printer, a fax machine, and so-called anMFP (Multi-Function Peripheral) that combines the functions of thesesmachines.

FIG. 1 is a plan view showing a configuration of an optical system of animage forming apparatus according to an embodiment of the presentinvention, FIG. 2 is a perspective view schematically showing theconfiguration of the optical system shown in FIG. 1, and FIG. 3 is aview schematically showing an internal configuration of a tandem typecolor image forming apparatus using the optical system shown in FIGS. 1and 2.

In FIG. 1, reference numeral 10 is a light scanning unit having apolygon mirror 11 at the center thereof. The polygon mirror 11 isrotated in the counterclockwise direction about a rotation axis 11 a. Asshown in FIG. 2, the polygon mirror 11 has a two-tier structureconstituted by an integrally formed upper mirror 111 and a lower mirror112, as shown FIG. 2.

A laser beam is projected onto the polygon mirror 11 through first andsecond light incident paths symmetrically disposed with respect to aplane including the rotation axis 11 a of the polygon mirror 11. Thatis, laser beams emitted from laser sources 12K and 12C are passedthrough a beam splitter 13L and projected on the polygon mirror 11;similarly, laser beams emitted from laser sources 12Y and 12M are passedthrough a beam splitter 13R and projected on the polygon mirror 11.

Laser beams emitted from the laser sources 12C and 12K enter the beamsplitter 13L and then emitted therefrom in parallel to each other;similarly, laser beams emitted from the laser sources 12M and 12Y areincident on the beam splitter 13R and then emitted therefrom in parallelto each other.

The laser beams emitted from the respective laser sources are passedthrough the light paths symmetrically running with respect to a planeincluding the rotation axis 11 a of the polygon mirror 11 respectivelyand enter the polygon mirror 11. The laser beams entering the polygonmirror 11 are then reflected by it. After that, the laser beams from thelaser sources 12K and 12C are passed through a first fθ lens 14L and asecond fθ lens 15L respectively, where scan angles of the laser beamsare corrected. The laser beams then enter a reflection mirror 16L andfolded by it. On the other hand, the laser beams from the laser sources12Y and 12M are passed through a first fθ lens 14R and a second fθ lens15R respectively, enter a reflection mirror 16R and folded by it.

The laser sources 12Y, 12M, 12C, and 12K project laser beams obtained bymodulating their output light with image data of Y (yellow), M(magenta), C (cyan), and K (black). As shown in FIG. 2, two laser beams(C, M) are projected onto the upper mirror 111 of the polygon mirror 11and other two laser beams (K, Y) are projected onto the lower mirror112. Assuming that a pair of laser beams (K, C) that is projectedthrough one incident path is referred to as first group and a pair oflaser beams (Y, M) that is projected through the other incident path isreferred to as second group, the laser beams of the first and secondgroups are reflected by the polygon mirror 11 in symmetrical directions.A rotation of the polygon mirror 11 allows the laser beams of the firstand second groups to be scanned in the direction denoted by dotted linesA and B, respectively.

As shown in FIG. 2, the first fθ lenses 14L, 14R, second fθ lenses 15L,15R, and reflection mirror 16L, 16R have two-tier structures. The laserbeams output from the laser sources C, M are passed through the upperlight paths; on the other hand, the laser beams output from the lasersources K, Y are passed through the lower light paths.

Although details will be described later, the laser beams folded by thereflection mirrors 16L, 16R expose photoconductor drums for respectivecolors (K, C, M, Y) through a not shown optical system (reflectionmirror, etc.).

Further, beam detector 17L, 17R are disposed at the portions in thevicinity of the start edges of the scanning operation of the laser beamsfor the reflection mirrors 16L, 16R, the portion being an overscanregion (a region out of the scan region in which the photoconductor drumis exposed for forming an image). The beam detector 17L, which isconstituted by a reflection mirror 18L and a light sensor 19L, detectsthe two laser beams (K, C) of the first group, and the beam detector17R, which is constituted by a reflection mirror 18R and a light sensor19R, detects the two laser beams (M, Y) of the second group.

It is possible to generate synchronization signal for synchronizing thelaser beams in the main scanning direction by utilizing detectionsignals (BD signals) from the beam detectors 17L, 17R. Each of the lightsensors 19L, 19R has two light detection devices optically andelectrically isolated from each other. By the light detection devices,the light sensor 19L independently detects the laser beams K, C, and thelight sensor 19R independently detects the laser beams Y, M.

As shown in FIG. 1, the light scanning unit 10 is symmetricallyconfigured with respect to a plane including the rotation axis 11 a ofthe polygon mirror 11. Assuming that the distance between the beamdetector 17L and reflection mirror 16L is L1, and distance between thebeam detector 17R and reflection mirror 16R is L2, L1 is smaller than L2(L1<L2).

Assuming that L1=L2 is satisfied, the laser beams enter the reflectionmirrors 18L and 18R simultaneously at the time when the polygon mirror11 is rotated, if manufacturing error is ignored. As a result, the beamdetector 17L and beam detector 17R detect the laser beams at the sametime.

When L1<L2 is satisfied as described above, the reflection mirror 18Rreceives the laser beam at an earlier timing than the reflection mirror18L does at the time when the polygon mirror 11 is rotated, with theresult that the beam detector 17R always detects the laser beam earlierthan the beam detector 17L does. That is, in the scanning optical systemhaving a symmetrical configuration, only the light sensors 19L and 19Rare shifted from the symmetrical position so that the light sensor 19Rreceives the laser beam at an earlier timing than the light sensor 19Ldoes. The difference between L2 and L1 is determined such that the beamdetector 17R always detects the laser beam earlier than the beamdetector 17L does in consideration of displacement of the fixed positionof optical elements constituting the scanning optical system at themanufacturing time, positional fluctuation of the optical elements dueto a change in environmental conditions such as temperature, a change inrotation balance resulting from micro vibration due to increase intemperature of the polygon mirror, and the like.

The positions of the beam detectors 17L and 17R are not limited to theshown example. For example, they may be disposed at the portions in thevicinity of the start edges of the scanning operation of the laser beamsfor the fθ lenses 15L, 15R. Also in this case, the difference between L2and L1 should be determined such that the beam detector 17R alwaysdetects the laser beam earlier than the beam detector 17L does. Thereason that the beam detector 17R should detect the laser beam earlierwill be described later in the description of circuit operation.

Although the beam splitters 13L, 13R are used to output the two laserbeams in parallel to each other, other optical systems such asreflection mirrors may be used in place of the beam splitters.

FIG. 3 schematically shows an internal configuration of a tandem typecolor image forming apparatus 20 utilizing laser beams folded by thereflection mirrors 16L, 16R. The image forming apparatus 20 has fourimage forming units 21K, 21C, 21M, 21Y in order to form respective colorimages of K (black), C (cyan), M (magenta), Y (yellow). The four imageforming units are arranged in the moving direction X of a transferpaper. The image forming units 21K, 21C, 21M, 21Y have photoconductordrums 22K, 22C, 22M, 22Y, respectively. The photoconductor drums 22K,22C, 22M, 22Y are arranged in a line with a predetermined interval alongthe moving direction X (sub scanning direction) of a transfer paper. Therotation axes of the photoconductor drums 22K, 22C, 22M, 22Y are madeparallel to the main scanning direction.

The image forming units 21K, 21C, 21M, 21Y have the same configuration.Here, the configuration of the image forming unit 21K will be describedas a representative. The image forming unit 21K includes a charger 23K,a development unit 24K, and a transfer unit 25K, which are disposedaround the photoconductor drum 22K. Similarly, the image forming units21M, 21C, 21Y have the chargers, development units, and transfer units.A feeding belt 26 is wound between a drive roller 27 and a driven roller28 and is rotated in the direction denoted by the arrow X (sub scanningdirection). Paper supply cassettes 29, 30 are provided below the feedingbelt 26.

A transfer paper is firstly fed to the image forming unit 21Y, where ayellow image is formed. The surface of the photoconductor drum 22Y isuniformly charged by the charger, exposed by the laser beam output fromthe laser source 12Y, which has been modulated by the yellow image data,and thereby an electrostatic latent image is formed. The electrostaticlatent image formed on the photoconductor drum 22Y is developed by thedeveloping unit and thereby a yellow toner image is formed on thephotoconductor drum 22Y. The toner image is transferred onto thetransfer paper by the transfer unit at the position (transfer position)where the photoconductor drum 22Y comes into contact with the transferpaper on the feeding roller 26 and thereby yellow image is formed on thetransfer paper. After completion of the transfer process, cleaning isapplied to the photoconductor drum 22Y to eliminate unnecessary tonerremaining on the surface thereof for the next image forming process.

The transfer paper on which yellow image has been transferred in theimage forming unit 21Y is fed to the image forming unit 21M by thetransfer belt 26. After that, images of magenta, cyan, black aresequentially transferred onto the transfer paper in the same manner.After the transfer of images of four colors has been completed, thetransfer paper is separated from the feeding belt 26 and dischargedoutside the apparatus.

The image forming apparatus 20 further includes a laser exposure unit 31utilizing laser beam scanning. The laser exposure unit 31 includes thelight scanning unit 10 shown in FIGS. 1 and 2 and performs scanningoperation in the main scanning direction to irradiate the surfaces ofthe photoconductor drums 22Y, 22M, 22C, 22K with laser beams for Y, M,C, K.

The image forming apparatus 20 further includes a color scanner section32. The color scanner section 32 has a transparent document table 33thereabove. The color scanner section 32 uses an image sensor to readout a color image of the document placed on the document table 33 andconverts the image into an electrical signal corresponding to threeprimary colors of R (red), G (green), B (blue).

The RGB signal is converted into color signals of Y (yellow), M(magenta), C (cyan), K (black), from which image data of C, M, K, Y aregenerated. The respective laser sources of the laser exposure unit 31are controlled based on image data of respective colors.

Further, in order to detect a supply state of a paper to be fed from thepaper supply cassettes 29, 30, a photosensor 34 that detects the startof the paper supply is provided near the driven roller 28. Thephotosensor 34 may be disposed at any position as long as it can detectthe start of the paper supply.

The laser sources of the laser exposure unit 31 can be controlled byimage data created by a personal computer (PC) in addition to the imagedata obtained by the color scanner section 32.

A configuration of an image processor in the image forming apparatusaccording to the embodiment of the present invention will next bedescribed.

FIG. 4 is a block diagram showing a configuration of an image processor40. The image processor 40 has a reading unit 41 that reads out an imagedata, an image data processor 42, page memory controllers 43L, 43R, andlaser control units 44L, 44R.

Note that the page memory controller 43L and laser control unit 44L are,for example, for cyan (C) or black (K); on the other hand, the pagememory controller 43R and laser control unit 44R are, for example, formagenta (M) or yellow (Y). For the shake of brevity, in the followingdescription, the page memory controller 43L and laser control unit 44Lare assumed to be units for C or K, and the page memory controller 43Rand laser control unit 44R are assumed to be units for M or Y.

The image data from the reading unit 41 is supplied through an I/F 46 toa bus line 401 which is connected to a controller (CPU) 47 and an imagememory 48. Further, the image data created by the PC 49 or the like issupplied through a LAN and I/F 50 to the bus line 401.

The laser control units 44L, 44R control the laser sources 61L, 61R. Tothe laser control units 44L, 44R, detection signals (BD signals) fromthe light sensors 19L, 19R of the beam detectors 17L, 17R are input.

The reading unit 41, which is constituted by, for example, the colorscanner section 32, uses a CCD or the like to photoelectrically convertan image on a document so as to output an RGB image data. The RGB imagedata is compressed and stored in the image memory 48. When the imagedata that has been stored in the image memory 48 is printed out, theimage data processor 42 expands the compressed image data, performscolor space conversion of the RGB image data into a YMCK signal,performs image quality correction such as gamma control or tonecorrection and, after that, transfers image data of respective colors tothe page memory controllers 43L, 43R.

In the block diagram shown in FIG. 4, image data of C (cyan) is outputto the page memory controller 43L and image data of M (magenta) isoutput to the page memory controller 43R, or image data of K (black) isoutput to the page memory controller 43L and image data of Y (yellow) isoutput to the page memory controller 43R. The reason for applying thecompression to image data when it is stored in the image memory 48 is toeffectively utilize the memory. After the data has been read out, theexpansion of the data is to be performed.

Image data transferred to the page memory controllers 43L, 43R arepulse-width modulated by the laser control units 44L, 44R, respectively.After that, the pulse-width modulated image data are output to the lasersources 61L, 61R and used for laser control. The laser source 61Lcorresponds to the laser source 12C or 12K shown in FIGS. 1 and 2, andthe laser source 61R corresponds to the laser source 12M or 12Y. Thelaser beams from the respective laser sources are passed through theoptical system shown in FIGS. 1 and 2 and irradiated onto thephotoconductor drums 22 of the image forming units 21C, 21M or imageforming units 21K, 21Y shown in FIG. 3. The page memory controllers 43L,43R read out image data in units of page from the image data processor42 and output the data to the laser control units 44L, 44R based ondetection signal BD (L) from the light sensor 19L. That is, the pagememory controllers 43L, 43R generate enable signal based onsynchronization signal (H sync) which is generated based on detectionsignal BD (L) and an output of the photosensor 34 that detects the startof the paper supply and transfer image data to the laser control units44L, 44R.

FIG. 5 shows a concrete configuration of the laser control units 44L,44R. The laser control unit 44L includes a line buffer memory 51L, amemory controller 52L that controls transfer of data to the line buffermemory 51L, a pulse width shaping circuit 53L that receives detectionsignal BD (L) from the light sensor 19L as an input, and a PWM (PulseWidth Modulation) circuit 54L that pulse-width modulates the data thathas been read out from the line buffer memory 51L. In FIG. 5, the laserbeams of C (cyan) and K (black) are detected by the sensor 19L, andlaser beams of M (magenta) and Y (yellow) is detected by the sensor 19R.

BD (L) signal detected by the light sensor 19L is input to the pulsewidth shaping circuit 53L, where the signal is shaped into a waveformwith a predetermined pulse width and then converted into synchronizationsignal H sync (L) whose timing is synchronized with the laser controlunit 44L. Here, in order to synchronize the timing of synchronizationsignal H sync (L) with the laser control unit 44L, pulse clock (P clock)to be used in the main part of the laser control unit 44L is input tothe pulse width shaping circuit 53L. Subsequently, synchronizationsignal H sync (L) and main clock signal (m Clock) are input to aflip-flop 57L. Main clock signal (m Clock) is a clock to be used in thepage memory controllers 43L, 43R. Synchronization signal H sync (L)whose timing has been readjusted by main clock signal (m Clock) is inputto the page memory controllers 43L, 43R. It should be noted thatsynchronization signal H sync (L) is input not only to the page memorycontroller 43L, but also to the page memory controller 43R.

Transmission of image data from the image data processor 42 is enabledwhile synchronization signal H sync (L) is transferred to the pagememory controllers 43L, 43R. Enable signal (DATA Enable) is generated inresponse to the input of synchronization signal H sync (L), and therebyimage data is loaded into the memory controller 52L in units of page. Asdescribed above, the page memory controllers 43L, 43R operate accordingto the common main clock signal (m Clock) and are synchronized withsynchronization signal H sync (L) generated by the flip-flop 57L.

Further, as described above, the main part of the laser control unit 44Loperates according to pulse clock (P clock) and pulse clock (P clock) issupplied to the memory controller 52L and PWM circuit 54L. Further, animage data (DATA C) of C (cyan) is transferred from the page memorycontroller 43L to the laser control unit 44L.

Enable signal (DATA Enable) to be transferred from the page memorycontroller 43L and input to the laser control unit 44L becomes Highlevel at the time when image data transfer is not enabled; whereas itbecomes Low level at the time when image data transfer is enabled. Thememory controller 52L receives Low enable signal that indicates theimage data transfer ready condition and reads parallel datacorresponding to one page of image data of C by sampling it for each oneline. The memory controller 52L converts the read image data into serialdata and writes the data corresponding to one line into the line buffermemory 51L.

The memory controller 52L sequentially reads out the image data in theline buffer memory 51L at a predetermined timing according to H sync (L)and transfers the read image data to the PWM circuit 54.

The data writing into the line buffer memory 51L is performed accordingto L sampling signal synchronized with synchronization signal H sync(L).

The time period in which color image data corresponding to one page istransferred, which is determined based on a paper supply start signalthat can be obtained from the photosensor 34 (FIG. 3), corresponds tothe exposure time period of image of C.

The PWM circuit 54L performs pulse width modulation in accordance withimage data level and supplies the laser source 61L for C (cyan) with alaser control signal. The PWM circuit 54L operates according to pulseclock (P clock) and outputs a signal whose pulse width is changed inaccordance to the value of picture element C within each cycle of pulseclock.

The laser control unit 44R has the same configuration as that of thelaser control unit 44L. The laser control unit 44R includes a linebuffer memory 51R, a memory controller 52R, a pulse width shapingcircuit 53R that receives detection signal BD (R) from the light sensor19R, and a PWM circuit 54R.

The pulse width shaping circuit 53R, to which detection signal BD (R)from the light sensor 19R and pulse clock (P clock) are input, generatessynchronization signal H sync (R) that has been shaped into a waveformwith a predetermined pulse width in accordance with the generationtiming of first clock signal after the input of BD (R) signal. The abovepoint is the same as that in the laser control unit 44L. The lasercontrol unit 44R differs from the laser control unit 44L in thefollowing point.

As described with reference to FIGS. 1 and 2, detection signal BD (R) ofthe light sensor 19R is allowed to be generated prior to the generationof the detection signal BD (L) of the light sensor 19L. Therefore, thephase of the synchronization signal H sync (R) generated from detectionsignal BD (R) is advanced relative to that of synchronization signal Hsync (L).

Synchronization signal H sync (L) generated by the laser control unit44L is input to the page memory controller 43R as a referencesynchronization signal to enable the image data processor 42 to send,for example, image data of M (magenta) and, subsequently, enable signal(DATA Enable) is supplied from the page memory controller 43R to thememory controller 52R of the laser control unit 44R. Further, pulseclock signal (P clock) is supplied to the memory controller 52R and, atthe same time, image data of M (DATA M) is loaded thereinto.

The memory controller 52R of the laser control unit 44R receives enablesignal of image forming instruction level (Low) from the page memorycontroller 43R, reads parallel data corresponding to one page of imagedata of M by sampling it, converts the read data into serial data, andwrites the data corresponding to one line into the line buffer memory51R. At the same time, the memory controller 52R sequentially reads outthe image data in the line buffer memory 51R at a predetermined timingaccording to H sync (R) and transfers the read image data to the PWMcircuit 54R. The data writing into the line buffer memory 51R isperformed according to R sampling signal synchronized withsynchronization signal H sync (R).

Although the time period in which image data of M corresponding to onepage is written into the laser control unit 44R is the same as that inwhich image data of C corresponding to one page is written into thelaser control unit 44L, the timing between the above two writingoperation is different. The time period in which color image datacorresponding to one page is transferred corresponds to the exposuretime period of image of M.

Image data of K (black) is processed using the same circuits in the pagememory controller 43L and laser control unit 44L. Image data of Y(yellow) is processed using the same circuits in the page memorycontroller 43R and laser control unit 44R.

An operation of the image forming apparatus of FIG. 5 according to theembodiment of the present invention will next be described withreference to FIG. 6. Logic signal in FIG. 6 is Low and active state.

The waveform shown in FIG. 6( a) represents synchronization signal Hsync (L) generated from detection signal BD (L) of the light sensor 19L.The waveform of FIG. 6( b) represents enable signal (DATA Enable) to beoutput from the page memory controller 43L and input to the memorycontroller 52L. Further, FIG. 6( c) represents sampling time period Lsampling in which the memory controller 52L of the laser control unit44L reads image data.

FIG. 6( d) represents synchronization signal H sync (R), which has beenobtained in the laser control unit 44R based on detection signal BD (R)from the sensor 19R, and FIG. 6( e) represents sampling time period Rsampling in which the memory controller 52R of the laser control unit44R reads image data.

As shown in FIG. 6( b), enable signal (DATA Enable) from the memorycontroller 43L is generated in response to generation of synchronizationsignal H sync (L) of FIG. 6( a). Image data (DATA L) and (DATA R) fromthe page memory controllers 43L, 43R are output to the laser controlunits 44L and 44R at the same timing as enable signal (DATA Enable).

The time period during which enable signal (DATA Enable) assumes Highlevel is time period during which transfer of image data from the pagememory controller is masked. When enable signal becomes Low level, imagedata can be transferred. When first synchronization signal H sync (L) isreceived after enable signal (DATA Enable) has become Low level,sampling of image data DATA (L) is started at timing t1 of FIG. 6( c)and data writing into the line buffer memory 51L is performed.

The sensor 19R for the laser control unit 44R always detects a laserbeam at an earlier timing than the sensor 19L for the laser control unit44L does, so that synchronization signal H sync (R) is generated priorto synchronization signal H sync (L) as shown in FIG. 6( d).

Therefore, as shown in FIG. 6( e), image data sampling time period Rsampling in the memory controller 52R precedes L sampling of FIG. 6( c)by time Δt. Sampling of image data DATA (R) from the page memorycontroller 43R is thus started and data writing into the line buffermemory 51R is performed.

FIG. 7 is a timing chart for explaining operation of an image formingapparatus of a conventional general type, in which beam detectors 17L,17R detect laser beams at the same timing. In this case, there is noproblem if BD (L), BD (R) are always generated at the same time.However, due to assembly error of the optical system, whether BD (L) orBD (R) is generated first is uncertain. Further, the rotation balance ofthe polygon mirror is changed by micro vibration of componentsconstituting a rotator due to increase in temperature of the polygonmirror, which may fluctuate detection timing of the BD sensor in somecases.

That is, assuming that sampling time period is set based on onesynchronization signal, for example, H sync (L), if the phase ofsynchronization signal H sync (R) is advanced relative tosynchronization signal H sync (L) as shown in FIG. 7( d), R sampling isslightly out of phase with L sampling as denoted by the solid line inFIG. 7( e). On the other hand, if the phase of synchronization signal Hsync (R) is retarded relative to synchronization signal H sync (ΔL), Rsampling is advanced relative to L sampling by as much as about one lineas denoted by the dotted line in FIG. 7( e). The shift between scanningpositions of the first and second laser beams makes color blurringnoticeable on a printed out image.

According to the present invention, even if the sampling time period isset based on one synchronization signal, for example, H sync (L), it ispossible to eliminate the above problem by allowing the detection of thesensor 19R to precede the detection of the sensor 19L. Therefore, it ispossible to prevent image degradation due to shift in the data writestart timing.

Another embodiment of the image forming apparatus according to thepresent invention will be described below with reference to FIG. 8. Inthe above embodiment, synchronization signal H sync (R) is allowed to begenerated prior to the generation of H sync (L) by positionalrelationship between the BD sensors 17L, 17R, that is, mechanicalstructure. On the other hand, in FIG. 8, synchronization signal H sync(R) is allowed to be generated prior to the generation of H sync (L) byelectrical processing.

FIG. 8 shows the main part of the laser control units 44L, 44R. Thelaser control unit 44L inputs detection signal BD (L) from the sensor19L to a Schmitt buffer 56L through a time constant circuit 55Lconstituted by a resistor R1 and a condenser C1, allows the timeconstant circuit 55L to integrate detection signal BD (L), outputs itthrough the Schmitt buffer 56L to delay the detection signal BD (L), andinputs the delayed detection signal BD (L) to the pulse width shapingcircuit 53L.

The laser control unit 44R inputs detection signal BD (R) from the BDsensor 19R to the pulse width shaping circuit 53R through the Schmittbuffer 56R. Other components are the same as those shown in FIG. 5.

In this example, synchronization signal H sync (L) generated based onthe delayed detection signal BD (L) is supplied, as a reference signal,to the page memory controllers 43L, 43R. Accordingly, the phase ofsynchronization signal H sync (R) is always advanced relative to thephase of synchronization signal H sync (L), so that sampling of imagedata is performed at the timing shown in FIGS. 6C and 6E. Thus, samplingtime period R sampling precedes L sampling by time Δt and data writinginto the line buffer memory 51R is started at a slightly earlier timing.By providing the adequate time constant circuit 55L, it is thus possibleto prevent the sampling timing from being shifted by about one line asshown in FIG. 7( e).

Although the time constant circuit 55L and Schmitt buffer 56L are usedas a delay means, it is possible to use any delay circuit as long as itcan set delay time such that the phase of synchronization signal H sync(R) is always advanced relative to the phase of synchronization signal Hsync (L) serving as a reference signal. However, if the delay time istoo large, the shift substantially corresponding to one line may occuras shown in FIG. 7( e). Therefore, phase difference At needs to be setto an adequate value. For example, a counter circuit that uses clocksignal (P Clock) to delay time corresponding to several clocks may beadopted. As clock signal (P Clock), clock signal of about 50 MHz isused.

As described above, according to the image forming apparatus of thepresent invention, in the case where the polygon mirror is used tosimultaneously generate a plurality of laser beams to perform exposurescanning operation for the photoconductor drums, it is possible tosuppress so called line misalignment in writing process of the laserbeams onto the photoconductor drums to thereby prevent imagedegradation.

In the above description, the phase of synchronization signal H sync (R)is advanced relative to the phase of synchronization signal H sync (L)and synchronization signal obtained based on detection signal BD (L)from the BD sensor 19L is supplied to the page memory controllers 43L,43R. Alternatively, however, synchronization signal obtained based ondetection signal BD (R) from the BD sensor 19R may be supplied to thepage memory controllers 43L, 43R.

Further, whether the phase of synchronization signal H sync (R) or Hsync (L) is advanced is not an essential problem. The phase ofsynchronization signal H sync (L) may be advanced relative to the phaseof synchronization signal H sync (R). The point is that the time-baserelation between synchronization signals H sync (R) and H sync (L) isnot allowed to be changed.

Although exemplary embodiments of the present invention have been shownand described, it will be apparent to those having ordinary skill in theart that a number of changes, modifications, or alterations to theinvention as described herein may be made, none of which depart from thespirit of the present invention. All such changes, modifications, andalterations should therefore be seen as within the scope of the presentinvention.

1. An image forming apparatus that controls a plurality of laser beamsbased on image data of different colors and performs exposure scanningoperation for a photoconductor drum to form an image, comprising: firstand second laser source sections that irradiate a polygon mirror withlaser beams from different directions; first and second light scanningsections that allow the polygon mirror to reflect the laser beams fromthe first and second laser source sections toward different optical axisdirections to scan the laser beams at a predetermined deflection angle;a first beam detector that is disposed at the beam incident end in thescanning direction of the first light scanning section and receives thefirst laser beam to detect the start of the scanning; a second beamdetector that is disposed at the beam incident end in the scanningdirection of the second light scanning section and receives the secondlaser beam to detect the start of the scanning at an earlier timing thanthe first beam detector does; and a laser controller that utilizes thedetection result of the first beam detector to set sampling start timingin common between the first and second image data, writes first andsecond image data in a memory based on the detection results of thefirst and second beam detectors, and controls the first and second lasersource sections based on the image data read out from the memory.
 2. Theimage forming apparatus according to claim 1, wherein the first andsecond beam detectors are disposed at an overscan region which is aregion out of the scan region in which the photoconductor drum isexposed for forming an image such that the second beam detector receivesthe second laser beam at an earlier timing than the first beam detectorreceives the first laser beam.
 3. The image forming apparatus accordingto claim 1, wherein the polygon mirror has upper and lower mirrorsurfaces, and the first and second laser source sections has a pluralityof laser sources that are vertically arranged and project laser beams ofdifferent colors on the mirror surfaces of the mirror, the laser beamsrunning in parallel to each other.
 4. The image forming apparatusaccording to claim 1, further comprising: a synchronization signalgeneration circuit that generates first and second synchronizationsignals corresponding to the first and second laser beams respectivelybased on the detection result of the first and second beam detectors,wherein sampling start timing is set in common between the first andsecond image data based on the first synchronization signal, and firstand second image data are written in a memory based on the first andsecond synchronization signals.
 5. The image forming apparatus accordingto claim 1, wherein the image forming apparatus is a tandem type imageforming apparatus that has a plurality of photoconductor drums to beexposed by laser beams emitted from a plurality of laser sources.
 6. Animage forming apparatus that controls a plurality of laser beams basedon image data of different colors and performs exposure scanningoperation for a photoconductor drum to form an image, comprising: meansfor detecting a reference point for main scanning operation of laserbeams with a beam detector and generating synchronization signalscorresponding to respective laser beams in order to control timing ofexposure operation performed using the laser beams; means for allowingone (second) synchronization signal of the synchronization signalscorresponding to the respective laser beams to be advanced relative toother (first) synchronization signal; a memory controller that utilizesan enable signal generated based on the first synchronization signal toset sampling start timing of plurality of image data and writes theimage data into a memory based on the corresponding synchronizationsignal; and laser control means for modulating a laser beam based on thecorresponding image data that has been read out from the memorycontroller.
 7. The image forming apparatus according to claim 6, whereinthe means for allowing the second synchronization signal to be advancedrelative to the first synchronization signal is constituted by a delaycircuit that delays the first synchronization signal serving as areference signal relative to the second synchronization signal.
 8. Theimage forming apparatus according to claim 6, wherein the laser controlmeans includes a PWM circuit that pulse-width modulates a laser beambased on the corresponding image data that has been read out from thememory controller.